The present invention relates to a process for minimizing the mold deposit problem frequently encountered when polyoxymethylene molding compositions are molded. Such deposits are detrimental to the formation of a quality molded article having smooth and uniform surface characteristics.
As is well known polyoxymethylene or polyacetal, is a thermoplastic resin which finds wide utility in the manufacture of shaped articles by injection molding or extrustion processes. Polyoxymethylene has many excellent mechanical properties which result in shaped articles characterized by their hardness, strength and toughness.
Polyoxymethylene resin, however, is subject to degradation particularly under the influence of heat, the amount of degradation being a factor of the method of preparation of the polyoxymethylene and the like. The degradation may occur for example, as the result of oxidative attack. The oxidative attack, which may lead to chain scission and depolymerization, is often retarded by the addition of antioxidants to the polyoxymethylene composition. Degradation is also believed to occur as the result of acidolytic cleavage of the polymer chain caused by acidic species present in the polymer. The acidic species may be acidic catalyst residues derived from catalysts used in the formation of the polymer or may be acetic acid generated from acetate end groups when a given chain, so stabilized, depolymerizes as a result of occasional oxidative or acidolytic chain scission. To assist in minimizing such degradation of polyoxymethylene especially during subsequent processing in the hot, or melt, state, "acid seavengers" are often admixed with the polymer composition. Albeit most commercially available polyoxymethylene is "prestablized" either by means of acetylation or hydrolysis treatments (see, for example, U.S. Pat. No. 3,839,267) or by the addition of additives such as the above-mentioned antioxidants and/or acid scavengers, it has been found that during the high-temperature molding, particularly, injection molding of the non-fiber reinforced resin an objectionable film, or mold deposit, commonly forms on the surface of the mold. The mold deposit, which can lead to surface defects on the molded resin, is generally believed to be of two types. One type of mold deposit is believed to be caused by the use of certain antioxidants which plate out on the mold surface. This type of mold deposit can be eliminated by using a less volatile antioxidant. The second type of mold deposit is believed to be caused by formaldehyde (generated, for example, as a result of the chain scission of the polyoxymethylene under the conditions of the molding process) condensing on the mold surface. The chain scission, in turn, is believed to be caused by acidic residues present in the polyoxymethylene and which have not been "cleaned up" by the prior stabilization treatments.
Although the thermal stabilization, i.e., stabilization against the effects of temperatures encountered in the melt state, of polyoxymethylene has heretofore been proposed in the art such as, for example, by the addition to the polyoxymethylene of amino substituted amides (U.S. Pat. No. 3,274,149), carbamates (U.S. Pat. No. 3,144,431), or severely hindered carbodiimides (British Pat. No. 993,600), such stabilization has either not been effective in removing the mold deposit tendency or results in undesirable discoloration of the polymer.
The physical blending of polyoxymethylene with thermoplastic resins, so as to improve the properties of a polycarbonate has also been proposed. Such blending has not been directed at improving the properties of the polyoxymethylene and has employed a relatively large quantity of the polycarbonate. Thus, for example, Miller, U.S. Pat. No. 3,646,159, discloses the blending of polyoxymethylenes, or polyacetals with a polycarbonate to improve the properties of the polycarbonate and to provide a polycarbonate mixture having improved resistance to environmental stress cracking and crazing. Although Miller suggests, inter alia, that the polyacetals may be employed in amounts of from 25 to 95 percent by weight based on the combined weight of the polycarbonate and polyacetal, the working examples are limited to blends containing a maximum of about 50 percent by weight of polyacetal. Goldblum, U.S. Pat. No. 3,290,261, discloses the blending of polycarbonate and up to about 20 percent polyacetal to obtain a foamed polycarbonate resin. Goldblum discloses that if the amount of polyacetal exceeds 20 percent, then the blend begins to lose the beneficial properties of the polycarbonate (Column 1, line 47).
Polycarbonates have also been disclosed as additives or modifiers or scavengers for polyester tire cord when added to the polyester prior to fiber formation (see, for instance, Rye et al, U.S. Pat. No. 3,563,847).
In light of this prior art it is an object of the present invention to provide a process for the preparation of an improved polyoxymethylene molding composition which forms reduced mold deposits upon molding.
It is a further object of the invention to provide a polyoxymethylene molding composition having high stability when subjected to the influence of heat and particularly when subjected to the conditions typically encountered during molding operations.
It is still another object of the present invention to provide an improved non-reinforced polyoxymethylene composition for an injection molding process.
Another object of the present invention is the provision for the preparation of a stabilized polyoxymethylene molding composition employing an aromatic polycarbonate additive, which stabilized polyoxymethylene is not undesirably discolored.
These and other objects of the invention will become apparent from the following summary and description of preferred embodiments.